The present invention relates to a method for cutting sheets of brittle materials into desired configurations or shapes utilizing a combined laser-scribing and laser-breaking technique. The present invention has particular applicability in cutting or separating brittle, non-magnetic sheets along curvilinear paths to produce substrates for use in the manufacture of magnetic and/or magneto-optical (MO) recording media.
Two techniques are conventionally employed for cutting or shaping a sheet of brittle material, such as a glass, amorphous glass, glass-ceramic or ceramic material, to form a sheet or substrate with a desired configuration or geometry. A first such conventional method involves mechanical scribing of the sheet employing a hard device, such as a diamond tip, to score the surface of the brittle material, which is then broken along the score line or pattern. The second of such conventional techniques involves laser-scribing. Currently employed laser-scribing differs from traditional high power (i.e.,  greater than 1 KW) laser-drilling/cutting and utilizes a lower power (i.e.,  less than 500 W) for achieving scribing with less material removal and better edge quality subsequent to breaking/separation. Such laser-scribing typically utilizes a continuous wave (xe2x80x9cCWxe2x80x9d) laser, such as a CO2 laser of 10.6 xcexcm wavelength, to heat a localized zone of a brittle material, such as an amorphous-glass sheet (similar to float glass), up to a temperature below the softening point of the material, and then immediately quenching the heated zone by applying a fluid coolant, e.g., a gas, such as air, a liquid, such as water, or a combination of a gas and a liquid, such as air/water.
In a typical process for laser-scribing an amorphous glass sheet, the output beam of a 10.6 xcexcm CW CO2 laser, or a high frequency pulse repetition rate 10.6 xcexcm CO2 laser, is re-shaped into a beam with an elongated spot shape, which beam is utilized in an unfocussed manner for locally heating the glass. The locally heated zone is then chilled by spraying thereon cool air or an air/liquid (e.g., air/water) mixture. When the localized heating/cooling process starts from a small surface defect or micro-crack made in the glass, e.g., by a means of a mechanical scriber or indenter, or by application of suitable laser pulses, the defect or micro-crack propagates to form a scribing line due to the combination of localized heating-quenching which initiates tiny surface cracks arising from compression-tension stress effects. The sheet of material is then separated, i.e., broken, along the scribing line by applying an external thermal or mechanical stress.
A conventional laser-scribing technique utilizing a low power CO2 laser is disclosed by Kondratenko in U.S. Pat. No. 5,609,284, wherein an elliptical target area is impinged with a beam of coherent radiation along the intended direction of the crack, while a stream of fluid coolant is directed at a point on the heated surface on the intended line of the crack. U.S. Pat. No. 6,259,058 B1 to Hoekstra discloses a modification of U.S. Pat. No. 5,609,284 wherein dual laser beams are utilized after cooling in order to assist separation along the laser-scribing line. Allaire et al. in U.S. Pat. No. 5,776,220 disclose a laser-scribing technique for brittle materials wherein the laser spot has an extremely elongated elliptical shape such that its major axis is greater than 20 mm to enable rapid scribing.
Conventional substrates for use in manufacturing magnetic recording media include various brittle materials, such as glasses, ceramics and glass-ceramics. In order to form annular disk-shaped substrates suitable for use in magnetic and/or magneto-optical (MO) recording media, two circular scribings must be performed with high precision, one defining the outer diameter (e.g., ranging from about 65 to about 95 mm, such as 84 mm) and one defining the inner diameter (e.g., ranging from about 20 to about 25 mm). However, applicability of current linear laser-scribing techniques, such as utilized with flat panels, to circular scribing for producing annularly-shaped substrates suitable for manufacture of disk-shaped magnetic and/or magneto-optical recording media, is limited, for at least the following reason: laser-scribing is very sensitive to variations of the glass material, including optical reflectivity of the surface, glass composition, surface and thickness uniformity, etc., resulting in that the CO2 laser-based scribing process requires very precise control of defect initialization, laser power distribution, and cooling stream. As a consequence, current laser-scribing technology of amorphous glass substrates is generally restricted to linear scribing.
Another drawback/disadvantage of conventional laser-scribing technology is associated with the methodology for separating/breaking the brittle substrate (e.g., of amorphous glass) subsequent to laser-scribing. Specifically, because of the nature of the localized heating/cooling of the laser-scribing process, and due to the formation of a compression layer on the surface of the amorphous glass sheet, the propagation of micro-cracks during the laser-scribing process occurs in the layer nearest the glass surface. As a consequence, the scribe line provided by a single laser beam at the surface of a glass surface is insufficiently deep, and application of additional mechanical force to the glass sheet is typically required during the laser-scribing process or subsequent thereto, disadvantageously resulting in edge defects, residual stresses, increased risk of cracking resulting in product loss (i.e., low yield), reduced product throughput, and poor cost-effectiveness arising from a requirement for complicated, thus expensive, processing.
More specifically, FIG. 1 shows an example of a linear laser-scribing process performed on a glass sheet for separating the latter into two segments, wherein a stationary, elongated, elliptically-shaped CO2 laser beam and a H2O/air cooling spray are successively supplied to a moving glass sheet along a substantially straight line of defect initialization markers (indicated by dark circles in the drawing) previously formed in the surface of the sheet, as by mechanical scribing/indentation or pulse laser irradiation, to form a laser scribe line (indicated by the dashed line in the drawing) within the glass sheet.
FIG. 2 illustrates an example of a similar, curvilinear laser-scribing process performed on a square or rectangular glass sheet for shaping the latter into an annular disk-shaped substrate, e.g., for use in the manufacture of disk-shaped, thin film magnetic and/or magneto-optical (MO) recording media. In this instance, the sheet is preliminarily provided with defect initialization markers (again indicated by dark circles in the drawing) arranged in a pair of concentric circles having diameters generally corresponding to the desired inner diameter (ID) and outer diameter (OD) of the medium, and proportionately-sized, arcuately extending, elliptically-shaped, stationary CO2 laser-scribing beams, along with a stationary H2O/air cooling spray, are utilized for scribing each of the inner and outer diameters along the corresponding circular lines of defect initialization markers while the sheet is rotated about a central axis to effect relative movement between the sheet and each of the laser-scribing beam +H2O/air spray combinations.
However, as indicated supra, because the thus-described conventional laser-scribing process: (1) involves localized heating/cooling, and (2) formation of compression layer on an amorphous glass surface, micro-crack formation occurs essentially only within the compression layer adjacent the glass surface. As a consequence, the laser scribing line which is formed is insufficiently deep, as shown in FIG. 3, and a post-scribing thermal-breaking step (refer to FIG. 4) is typically required to effect separation along the scribing line, in which step a portion of the sheet on one side of the scribing line is heated, while another portion of the sheet on the opposite side of the scribing line is chilled, causing the scribing line to propagate across substantially the entire thickness of the sheet, thereby facilitating separation.
Because conventional CO2 laser-based scribing techniques can scribe glass substrates to a depth of only about 0.1-0.2 mm, when such conventional CO2 laser-based scribing is utilized for shaping glass sheets for magnetic and for MO recording media with a current minimum thickness of about 0.7 mm, a significant fraction of the thickness of the glass material, e.g., at least about 0.3-0.5 mm, would be required to be broken in the above-described post-scribing thermal-breaking step. In cases where the substrate is of greater thickness, or where curvilinear scribing + breaking is to be effected, a pair of laser beams, with at least one laser beam adapted for adjustable offset vis-a-vis the other laser beam, may be utilized for scribing the upper and lower surfaces, respectively, as illustrated in FIG. 5. As shown in more detail in FIG. 6, removal of the center portion of the sheet (e.g., by post-scribing thermal treatment) for forming the ID of the disk is facilitated by presence of the offset alignment of the laser-scribing lines on opposite sides of the sheet.
However, the post-scribing thermal-breaking step is slow, complicated, and of inconsistent quality, leading to product loss (i.e., reduced yield) and additional expense, ultimately resulting in poor cost-effectiveness. Moreover, the heating/chilling aspect of the process is difficult to perform with thicker glass sheets.
In view of the above-described disadvantages, drawbacks, and difficulties associated with utilization of conventional laser-scribing + post-scribing thermal-breaking technology for separating/shaping brittle substrates, there exists a clear need for improved method and apparatus for laser-scribing and shaping brittle substrates along a desired path or contour, such as a curvilinear path, particularly a substantially circular path. Further, there exists a particular need for improved methodology and apparatus for laser-scribing and shaping brittle materials, such as glasses, ceramics and glass-ceramics, along substantially circular paths to form annular disk-shaped substrates for use in manufacturing magnetic and MO recording media.
The present invention, therefore, addresses and solves the above-described drawbacks, disadvantages, difficulties, and shortcomings of the conventional methodologies and instrumentalities for performing laser-scribing as part of a process for shaping brittle substrates. According to the invention, a laser-based process and apparatus is provided which is especially well-adapted for performing combined scribing and post-scribing separation/shaping of brittle substrates, such as glass sheets, into a desired shape/contour, e.g., into annular-shaped disks with inner and outer diameters for use in the manufacture of magnetic and/or MO recording media, which methodology and apparatus provide a simple, readily controllable manufacturing process with increased product throughput and cost-effectiveness.
An advantage of the present invention is an improved method of shaping a sheet of brittle material by segmenting.
Another advantage of the present invention is an improved method of separating a sheet of a brittle material into portions by means of combined laser-scribing + laser-breaking.
Yet another advantage of the present invention is an improved method of shaping a sheet of brittle material along concentric inner and outer circular paths to form an annular disk.
A further advantage of the present invention is an improved apparatus for shaping a sheet of brittle material by segmenting.
A still further advantage of the present invention is an improved apparatus for separating a sheet of a brittle material into portions by means of combined laser-scribing + laser-shaping.
A yet further advantage of the present invention is an improved apparatus for shaping a sheet of brittle material along concentric inner and outer circular paths to form an annular disk.
Additional advantages and other features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to an aspect of the present invention, the foregoing and other advantages are obtained in part by a method of segmenting a sheet of a brittle material, comprising sequential steps of:
subjecting the sheet to a first laser-based process to form a laser-scribing line extending from a first surface of the sheet to a first depth below the first surface;
subjecting the sheet to a second laser-based process to form a laser-breaking line extending from the laser-scribing line at the first depth below the first surface to a second, greater depth below the first surface; and
segmenting the sheet along the thus-formed combined laser scribing + laser-breaking line, wherein the first laser-based process to form the laser-scribing line comprises sequential steps of:
irradiating selected portions of the first surface of the sheet to form locally heated portions; and
cooling the locally heated portions.
According to embodiments of the present invention, the method comprises steps of:
(a) providing a sheet of a brittle material having first and second opposing surfaces separated by a thickness;
(b) providing an apparatus comprising at least a first combined laser-scribing/laser-breaking assembly including, in spaced-apart relation and in the recited order:
(i) a first source of laser radiation adapted for irradiating selected portions of the first surface of the sheet of brittle material with a first wavelength laser beam for forming the laser-scribing line therein to the first depth below the first surface;
(ii) a source of fluid coolant adapted for cooling the selected portions of the first surface of the sheet of brittle material subsequent to the irradiation by the first source of laser radiation; and
(iii) a second source of laser radiation adapted for irradiating the selected portions of the first surface of the sheet of brittle material with a second wavelength laser beam for forming therein the laser-breaking line extending from the laser-scribing line at the first depth below the first surface to the second, greater depth below the first surface; the apparatus further comprising:
(iv) means for effecting relative movement between the first surface of the sheet and the at least one combined laser-scribing/laser-breaking assembly;
(c) subjecting the first surface of the sheet to processing by the at least one combined laser-scribing/laser-breaking assembly, while effecting relative movement therebetween, to thereby form at least one combined laser-scribing + laser-breaking line having a predetermined depth below the first surface and extending along a predetermined path; and
(d) segmenting the sheet along the at least one combined laser-scribing + laser-breaking line extending along the predetermined path.
In accordance with embodiments of the present invention, step (a) comprises providing a sheet of brittle material including a plurality of defect initialization markers formed in the first surface thereof, the plurality of defect initialization markers extending along at least one line corresponding to the predetermined path of the at least one combined laser-scribing + laser-breaking line formed in step (c), and according to embodiments of the present invention, the plurality of defect initialization markers are formed in the first surface of the sheet by mechanical scribing, mechanical indentation, or pulsed-laser treatment.
According to certain preferred embodiments of the present invention, step (a) comprises providing a sheet of a brittle material selected from the group consisting of glass, amorphous glass, ceramics, and glass-ceramics; step (b) comprises providing an apparatus wherein the first wavelength laser beam from the first source (i) is of a longer wavelength than the second wavelength laser beam from the second source (iii), e.g., step (b) comprises providing an apparatus wherein the first source (i) of laser radiation is a CW CO2 laser providing a first laser beam having a cross-sectional shape in the form of a first elongated ellipse, and the second source (iii) of laser radiation is a pulsed UV laser providing a second laser beam having a cross-sectional shape in the form of a second elongated ellipse, the first ellipse having a greater length than the second ellipse.
In accordance with further preferred embodiments of the present invention, step (b)) comprises providing an apparatus wherein the source of fluid coolant (ii) is a source of H2O/air, and further comprises providing an apparatus wherein the means (iv) for effecting relative movement between the first surface of the sheet and the at least one combined laser-scribing/laser-breaking assembly comprises means for moving the sheet while maintaining the at least one combined laser-scribing laser-breaking assembly stationary, thereby to form the at least one combined laser-scribing + laser-breaking line extending along a said predetermined path.
According to embodiments of the present invention, step (c) comprises moving the first surface of the sheet to form therein at least one combined laser-scribing + laser-breaking line extending along at least one predetermined linear, curvilinear, or linear-curvilinear path; and according to certain preferred embodiments of the present invention, step (c) comprises rotating the first surface of the sheet about a central axis to form therein at least one combined laser-scribing + laser-breaking line extending along at least one predetermined circular path.
In accordance with further embodiments of the present invention, step (b) comprises providing an apparatus comprising a pair of radially spaced-apart, combined laser-scribing/laser-breaking assemblies each arranged in an arc around a central axis; step (c) comprises moving the first surface of the sheet about the central axis while maintaining each of the pair of combined laser/scribing/laser-breaking assemblies stationary to form therein a concentric pair of combined laser-scribing + laser-breaking lines extending along a pair of predetermined circular paths; and step (d) comprises segmenting the sheet along each of the concentric pair of combined laser-scribing + laser-breaking lines to form an annular disk-shaped substrate.
According to still further embodiments of the present invention, step (b) comprises providing an apparatus comprising another first source of laser radiation (i) adapted for irradiating selected portions of the second surface of the sheet of brittle material with the first wavelength laser beam for forming a laser-scribing line therein in substantial vertical registry with the combined laser-scribing + laser-breaking line.
Another aspect of the present invention is an apparatus for performing combined laser-scribing and laser-breaking of a sheet of a brittle material, comprising:
(a) at least one combined laser-scribing/laser-breaking assembly including, in spaced-apart relation and in the recited order:
(i) a first source of laser radiation adapted for irradiating selected portions of a first surface of the sheet of brittle material with a first wavelength laser beam for forming a laser-scribing line therein extending from the first surface to a first depth below the first surface;
(ii) a source of fluid coolant adapted for cooling the selected portions of the first surface of the sheet subsequent to the irradiation by the first source of laser radiation; and
(iii) a second source of laser radiation adapted for irradiating the selected portions of the first surface of the sheet with a second wavelength laser beam for forming therein a laser-breaking line extending from the first depth below the first surface to a second, greater depth below the first surface to thereby form a combined laser-scribing + laser-breaking line; and
(b) means for effecting relative movement between the first surface of the sheet and the at least one combined laser-scribing/laser-breaking assembly to form at least one combined laser-scribing + laser-breaking line extending along a predetermined path.
According to certain embodiments of the present invention, the at least one combined laser-scribing/laser-breaking assembly (a) comprises: a first source (i) of laser radiation in the form of a CW CO2 laser providing a first laser beam having a cross-sectional shape of a first elongated ellipse; a source of H2O/air as the source of fluid coolant (ii); and a second source (iii) of laser radiation in the form of a pulsed UV laser providing a second laser beam of shorter wavelength than the first laser beam and having a cross-sectional shape in the form of a second elongated ellipse, the first ellipse having a greater length than the second ellipse; the means (b) for effecting relative movement between the first surface of the sheet and the at least one combined laser-scribing/laser-breaking assembly comprises means for moving the sheet while maintaining the at least one combined laser-scribing/laser-breaking assembly stationary, thereby to form the at least one combined laser-scribing + laser-breaking line extending along a predetermined linear, curvilinear, or linear-curvilinear path.
In accordance with further embodiments of the present invention, the apparatus comprises a pair of radially spaced-apart, combined laser-scribing/laser-breaking assemblies (a) each arranged in an arc around a central axis; and means (b) for effecting relative movement between the first surface of the sheet and the pair of combined laser-scribing/laser-breaking assemblies comprises means for rotating the sheet around the central axis while maintaining the pair of combined laser-scribing/laser-breaking assemblies stationary, to form in the sheet a concentric pair of combined laser-scribing + laser-breaking lines extending along a pair of predetermined circular paths.
According to still other embodiments of the present invention, the apparatus further comprises at least one additional first source of laser radiation (i) adapted for irradiating selected portions of a second, oppositely facing surface of the sheet of brittle material with a first wavelength laser beam for forming at least one laser-scribing line therein in substantial vertical registry with a the at least one combined laser-scribing + laser-breaking line formed by the at least one combined laser-scribing/laser-breaking assembly.
Yet another aspect of the present invention is an apparatus for performing combined laser-scribing and laser-breaking of a sheet of a brittle material, comprising:
(a) means for performing combined laser-scribing + laser-breaking of a sheet of brittle material; and
(b) means for moving the sheet of brittle material relative to means (a) to form therein at least one combined laser-scribing + laser-breaking line extending along a predetermined path.
Additional advantages and aspects of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein embodiments of the present invention are shown and described, simply by way of illustration of the best mode contemplated for practicing the present invention. As will be described, the present invention is capable of other and different embodiments, and its several details are susceptible of modification in various obvious respects, all without departing from (he spirit of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as limitative.